ipa: ipu3: awb: Introduce Black Level Correction

The pixels output by the camera normally include a black level, because
sensors do not always report a signal level of '0' for black. Pixels at
or below this level should be considered black and to achieve that, we
need to substract an offset to all the pixels. This can be taken into
account by reading the lowest value of a special region on sensors which
is not exposed to light. This provides a substracting factor to be
able to adjust the expected black levels in the resulting images.

For a camera outputting 10-bit pixel values (in the range 0 to 1023) a
typical black level might be 64. It is a fixed value, obtained by
capturing a raw frame with minimum exposure and gain fixed to 1.0 while
covering the sensor (the darker the better). We consider it good enough
as a very first approximation, until we measure it during a tuning
process and include it in a configuration file

Signed-off-by: Jean-Michel Hautbois <jeanmichel.hautbois@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

1 files changed, 67 insertions, 0 deletions

diff --git a/src/ipa/ipu3/algorithms/blc.cpp b/src/ipa/ipu3/algorithms/blc.cpp
new file mode 100644
index 00000000..78ab7bff
--- /dev/null
+++ b/src/ipa/ipu3/algorithms/blc.cpp
@@ -0,0 +1,67 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2021, Google inc.
+ *
+ * blc.cpp - IPU3 Black Level Correction control
+ */
+
+#include "blc.h"
+
+#include <string.h>
+
+/**
+ * \file blc.h
+ * \brief IPU3 Black Level Correction control
+ */
+
+namespace libcamera {
+
+namespace ipa::ipu3::algorithms {
+
+/**
+ * \class BlackLevelCorrection
+ * \brief A class to handle black level correction
+ *
+ * The pixels output by the camera normally include a black level, because
+ * sensors do not always report a signal level of '0' for black. Pixels at or
+ * below this level should be considered black. To achieve that, the ImgU BLC
+ * algorithm subtracts a configurable offset from all pixels.
+ *
+ * The black level can be measured at runtime from an optical dark region of the
+ * camera sensor, or measured during the camera tuning process. The first option
+ * isn't currently supported.
+ */
+
+BlackLevelCorrection::BlackLevelCorrection()
+{
+}
+
+/**
+ * \brief Fill in the parameter structure, and enable black level correction
+ * \param context The shared IPA context
+ * \param params The IPU3 parameters
+ *
+ * Populate the IPU3 parameter structure with the correction values for each
+ * channel and enable the corresponding ImgU block processing.
+ */
+void BlackLevelCorrection::prepare([[maybe_unused]] IPAContext &context,
+			      ipu3_uapi_params *params)
+{
+	/*
+	 * The Optical Black Level correction values
+	 * \todo The correction values should come from sensor specific
+	 * tuning processes. This is a first rough approximation.
+	 */
+	params->obgrid_param.gr = 64;
+	params->obgrid_param.r  = 64;
+	params->obgrid_param.b  = 64;
+	params->obgrid_param.gb = 64;
+
+	/* Enable the custom black level correction processing */
+	params->use.obgrid = 1;
+	params->use.obgrid_param = 1;
+}
+
+} /* namespace ipa::ipu3::algorithms */
+
+} /* namespace libcamera */